Liquid developer for electrophotography

ABSTRACT

A liquid developer for use in electrophotography, wherein a toner comprising pigment particles and a resinous layer coated on said pigment particles is dispersed in a carrier liquid of high electric resistance and low dielectric constant, said developer being characterized by the fact that the pigment particles are coated with a resinous layer consisting of at least two layers of which the first layer is directly coated on the particles and is comprised of a resin insoluble in said carrier liquid while the outermost layer comprises a resin capable of somewhat swelling up in said carrier liquid.

United States Patent Machida et a].

[451 June6, 1972 [54] LIQUID DEVELOPER FOR ELECTROPHOTOGRAPHY [72] Inventors: Hazime Machida; Zenjiro Okuno, both of Tokyo, Japan [73] Assignee: Kabushiki Kaisha Ricoh, Tokyo, Japan [22] Filed: June 25, 1969 21 App]. No.: 836,246

York ..252/62.1 Shinohara et al..

3,438,904 4/1969 Wagner ..252/62.l 3,140,175 7/1964 Kaprelian. .....252/62.1 3,080,318 3/1963 Claus ..252/62.1

3,080,251 3/1963 Claus ..252/62.l 3,080,250 3/1963 Claus ..252/62.l 2,899,335 8/1959 Straughan ..253/62.1

Primary Examiner-George F. Lesmes Assistant Examiner.l. P. Brammer Att0rney--Woodhams, Blanchard and Flynn [57] ABSTRACT A liquid developer for use in electrophotography, wherein a toner comprising pigment particles and a resinous layercoated on said pigment particles is dispersed in a carrier liquid of high electric resistance and low dielectric constant, said developer being characterized by the fact that the pigment particles are coated with a resinous layer consisting of at least two layers of which the first layer is directly coated on the particles and is comprised of a resin insoluble in said carrier liquid while the outermost layer comprises a resin capable of somewhat swelling up in said carrier liquid.

6 Claims, 3 Drawing Figures PATENTEnJun 6 I972 FIG.

FIG. 3

LIQUID DEVELOPER FOR ELECTROPIIOTOGRAPHY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid developer for electrophotography which is prepared by uniformly dispersing, in a carrier liquid, a toner provided with two or more resinous layers of different solubility in said carrier liquid, said layers being coated on the surfaces of pigment particles.

2. Description of the Prior Art Liquid developers in the prior art have been obtained by dispersing pigment particles, together with an additive such as a resin capable of controlling the polarity of pigment particles so as to impart to them dispersibility in the carrier liquid or stability to the toner, in a highly insulating carrier liquid to thereby cause said pigment particles to adsorb the resin within the carrier liquid and form a toner. Such a liquid developer has heretofore been manufactured by dispersing pigment particles, such as Carbon Black, in a carrier liquid, e.g., an organic solvent such as paraffinic hydrocarbon, upon kneading and pulverizing them together with a resin such as alkyd resin employed as a polarity-controlling agent or a dispersion stabilizer. In order to effect development by means of the thus obtained liquid developer, a copying material having an electrostatic latent image is immersed in said liquid developer to thereby cause the toner to be charged with positive or negative polarity so as to generate electrophoresis corresponding to the polarity of the latent image area and to adhere onto the surface of the photo-sensitive layer, whereby there is formed a visible image.

Since the liquid developers in the prior art are prepared by mere dispersion of pigment particles together with a specific additive in a carrier liquid as stated above, for some period of time subsequent to the manufacture thereof, such matters as the resin are relatively firmly adsorbed onto the pigment particles, the toner displays a distinct polarity, and a sufficient dispersibility thereof is maintained, but, with the passage of time, the adsorbability of the pigment particles deteriorates gradually, the adsorbed resin comes off the pigment particles, and the polarity of the toner becomes indistinct or the toner precipitates, whereby hampering the developing efficiency as compared with the initial stage subsequent to manufacture thereof. The root cause for deterioration of these qualities is the co-existence of the toner and the extra resin not adsorbed onto pigment particles within the carrier liquid, and the existence of such extra resin causes the insulating property of the carrier liquid to deteriorate or the electric charge of the toner to change, and, further in case of copying, said resin adheres to the electrostatic latent image area to lower the electric charge thereof, resulting in a decrease in the amount of adhered toner, and the copied image is not sufiiciently concentrated, i.e., distinct. Besides, the amount of resin adsorbed onto pigment particles within the carrier liquid is comparatively small and the adhesive power of the toner for the photosensitive layer surface of the copying material is weak. Further, when the copied image obtained was utilized as an offset master or transferred, it could hardly produce a satisfactory effect.

SUMMARY OF THE INVENTION The present invention is intended to provide a liquid developer composition for use in electrophotography by mean of dispersing, in 1,000 parts by weight of a highly insulating organic solvent, from l-5O parts by weight of a toner comprised of pigment particles coated with multiple layers of resins having different solubilities in the carrier liquid on the basis of the judgement that the above stated drawbacks of liquid developers in the prior art are attributable to the composition of the toner per se. The electrophotography as referred to in the present invention means producing a copy by converting an electrostatic latent image into a visible image after forming a latent image consisting of electrostatic charge on an appropriate electrostatic charge carrier. The foregoing liquid developer is applicable to the development of such electrostatic latent image.

The following is the concrete explanation of the present invention with reference to the accompanying drawing.

FIGS. 1, 2 and 3 of the drawing respectively are enlarged cross-sectional views of the structure of some examples of the toner for the liquid developer for electrophotography according to the present invention.

FIG. 1 and FIG. 3 show a toner having two resinous layers while FIG. 2 shows a toner having three resinous layers. The toner is respectively provided with a first resinous layer 2 formed directly on the surface of a pigment particle l, and on the top of said first resinous layer there is provided a second resinous layer, which corresponds respectively to the outermost resinous layer 3 in relation to the pigment particle in FIG. 1, and to the middle resinous layer 3a in FIG. 2, and to the resin-dispersed layer 31;, viz., the outermost layer in FIG.

' 3. In FIG. 2, there is further provided the outermost resinous layer 4. Further, in each toner, the solubilities of the resins contained in respective resinous layers thereof in the carrier liquid differs from one another. The resinous layer closest to the pigment particle preferably is insoluble in the carrier liquid while the outermost resinous layer is capable of somewhat swelling up therein. Accordingly, the first resinous layer 2, for instance, of each toner and the second resinous layer 3a of FIG. 2 are insoluble, whereas the second resinous layer 3 and 3b, respectively, of FIG. 1 and FIG. 3 and the third resinous layer 4 of FIG. 2 are capable of somewhat swelling up in the carrier liquid. Therefore, by dispersing these toners in an appropriate carrier liquid, there can be obtained the liquid developer for electrophotography according to the present invention.

In other words, the present invention relates to a liquid developer for electrophotography which is characterized in that a toner comprising pigment particles coated with two or more resinous layers of different solubilities in the carrier liquid is dispersed in a highly insulating carrier liquid. To be more precise, the resin coated directly onto the pigment particles is insoluble in the carrier liquid while the resin contained in the outermost resinous layer somewhat swells up in the same and the resin of intermediary layer(s) other than the foregoing two layers is possessed of either of these properties, and is preferably insoluble in the carrier liquid.

In preparing the toner according to the present invention, for instance, a toner comprising two or three resinous layers as shown in FIG. 1 and FIG. 2, 1 part by weight of pigment is mixed with 1 part by weight preferably 3 parts by weight of the selected 1st resin and is processed for thermal fusion of kneading upon adding a small amount of an appropriate solvent. Then, after cooling or evaporation of solvent, the grains obtained are pulverized to an approximate grain size of less than mesh.

Subsequently, 2 parts by weight of the thus pulverized particles and 1 part by weight of the second resin, selected as the resin for use in forming the outermost resinous layer in case of the two-layered toner and for use in forming the middle resinous layer in case of the three-layered toner, are mixed and then processed for either thermal fusion or kneading upon adding a small amount of an appropriate solvent which will not cause the first resinous layer formed in advance to dissolve or swell up. The thus kneaded mixture is pulverized, after cooling or solvent evaporation in the same way as in the foregoing, whereby there is obtained either the desired toner when the two-layered toner is being produced or the particle provided with the middle layer, viz., the 2nd resinous layer when the three-layered toner is being produced. Further, when the three-layered toner is being produced, by processing 2 parts by weight of the aforesaid mixed particles and 1 part by weight of the third resin selected for use in forming the outermost layer according as the treatment applied at the step of forming the outermost layer in case of the two-layered toner, a toner provided with three resinous layers can be formed. When preparing the toner provided with a resin dispersed layer as the outermost layer 3b such as shown in FIG. 3, it will suffice to coat pigment particles having the first resinous layer of the preliminary stage with a solution prepared by dispersing the second resin in an appropriate solvent, by means of spray drying and the like.

Through the uniform dispersion of the toner prepared as above in an appropriate organic solvent having a high insulating property (such as paraffinic or isoparaffinic hydrocarbon or a mixture thereof having an intrinsic volume resistivity of more than lO'Q. cm), a desired liquid developer for electrophotography can be obtained.

Though the pigment particles, resins and carrier liquid applicable in the present invention are the same as that employed for conventional liquid developers for use in electrophotography, care should be taken in selecting and combining these materials in order to make the toner and carrier liquid function effectively. However, so long as this requirement in met, it is at ones option to either select the pigment particles and resin to the carrier liquid or select the carrier liquid to the pigment particles and resin, providing that the resin'to be selected is preferred to have its second order transition point above 40 C. in order to avoid flowing of the toner at the time of fixing. Suitable combinations of materials in due consideration of the foregoing are as shown in examples hereunder, and the materials applicable in these combinations are enumerated as follows:

As for the pigment particles, there can be used Carbon Black (C.I.No. 77266), Spirit Black (C.I.No. 50415), Aniline Black (C.I.No. 50440), Alkali Black (C.I.No. 42750), Cyanine Blue (C.I.No. 26410), Fast Yellow (C.I.NO. 13135), Fast Red (C.I.No. 37125), Benzidine Orange (C.I.No. 21 1 l), Methylene Blue (C.I.No. 25015), etc.

As for the resin for the 1st layer to coat pigment particles directly, there can be used styrene, styrene-butylmethacrylate copolymer (a molar ratio of 7 3), styrene-laurylmethacrylate copolymer (a molar ratio of 9 l), styrene-hydroxymethacrylate copolymer(a molar ratio of 6 4), styrene-butadiene copolymer (a molar ratio of 8 2) as well as commercial resins, e.g., methylmethacrylate butylmethacrylate copolymers such as BR 60, BR 70, BR 75, BR 80, BR 85, BR 95 (manufactured by Mitsubishi Rayon Co. Ltd.), etc., and these .are respectively employed in the proportion of 0.5-10 parts by weight, based on pigment particles.

As for the resin to be employed for the outermost layer, there are styrene-laurylmethacrylate copolymer (a molar ratio of 1. l), styrene-butadiene copolymer (a molar ratio of l l), styrene-butylacrylate-acrylic acid copolymer (a molar ratio of 3 7 I) as well as commercial butylmethacrylate homopolymer such as BR 65, BR 90, BR 121, BR 122, BR 123, BR 101 BR 105 and BR 106 (products manufactured by Mitsubishi Rayon Co. Ltd.) vinyltoluene vinylacetate copolymer (manufactured by the Goodyear Tire & Rubber Co. under the trade name of Solprene), phenol-modified alkyd resin such as Nikanol AM, Nikanol S-IOI (products manufactured by Japan Gas-Chemical Co. Inc.), laurylmethacrylate hydroxymethacrylate copolymer (a molar ratio of 3:1 styrene-butadiene copolymer such as Pliolite AC, Pliolite ACL and Pliolite VTAC (products manufactured by the Goodyear Tire & Rubber Co.), and these are respectively employed at the proportion of l-2O parts by weight of the pigment particles. As to the respective intermediary layers, the 1st resin or the resin for the outermost layer is employed, and the appropriate amount is in the range of 0.5-l0 parts by weight of pigment particles.

As for the carrier liquid, there can be used paraffinic or isoparaffinic hydrocarbons or a mixture thereof such as naphtha, lsopar H (manufactured by ESSO Standard Sekiyu K.K.), etc.

The liquid developer according to the present invention has the feature that the polarity of the toner will not change nor will the dispersibility thereof deteriorate, and, accordingly, the

same developing efficiency as at the initial stage subsequent to the manufacture thereof can be always maintained satisfactorily because the resin never comes off the toner particles. Moreover, there being no surplus particles other than the toner particles within the carrier liquid, there is no fear of causing deterioration of the insulation resistance of the carrier liquid or deteriorations of the electric charge as well as the electrophoresis of the toner particles. Even more remarkable is that all the toner particles have a uniform size so that they are uniformly charged in the liquid developer, corresponding to the size thereof. Accordingly, they generate so rapid an electrophoresis towards the potential existing at a latent image area or a non-exposed-to-light area, which is electrostatically charged, that they adhere firmly; however, they do not do so towards the faint potential remaining at a non-latent image area or a exposed-to-light area. In other words, they adhere only along the potential line at the higher part of the electrostatic charge density capable of generating electrophoresis, but in the other lower part of said density they do not generate sufiicient electrophoresis in order to adhere thereto.

Accordingly, the liquid developer of the present invention is also characterized in that the toner particles adhere selectively onto an electrostatic latent image area corresponding to the electric charge thereof. In this respect, the toner used in the present invention is a new and improved type of charge selective toner compared with the conventional ones, and accordingly the occurrence of any stained background (nonimage area) at the time of copying can be avoided.

Further, the outermost resinous layer coated on the surface of pigment particles being somewhat swollen up in the carrier liquid, its adhesive property to the photo-sensitive layer of the copying material is very much strengthened. Not only that, the toner being composed of relatively many layers of resin, the visible image obtained through development is ready to be transferred if an appropriate solvent is selected, and further can be utilized as an excellent offset-printing master by employing an appropriate hydrophobic resin for the resinous layer of the toner.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1, FIG. 2 and FIG. 3 of the accompanying drawing are enlarged cross-sectional views showing the structure of examples of the toner to be employed for the liquid developer for electrophotography according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 A mixture of 7 parts by weight of styrene and 3 parts by weight of butylmethacrylate was subjected to bulk polymerization at a temperature of -l00 C. for 5 hours under a nitrogen current in the presence of azobis-isobutyronitrile, a reaction initiator, whereby there was obtained a styrene-butylmethcrylate copolymer (having a composition ratio of styrene resin 7: butylmethacrylate resin 3). Next, 1 part by weight of Spirit Black and 4 parts by weight of the foregoing copolymer were mixed with 5 parts by weight of toluene. The grains obtained upon evaporation of toluene were pulverized to a grain size of about mesh with a ballmill, whereby there were prepared fine grains comprising pigment particles coated with the 1st resinous layer. Then, 5 parts by weight of the thus obtained fine grains, 2 parts by weight of butylmethacrylate homopolymer (manufactured by Mitsubushi Rayon Co. Ltd. and sold under the name Standard 101 as the second resin and 3 parts by weight of toluene were mixed. The grains obtained upon evaporation of toluene were pulverized to about 150 mesh with a ball-mill, whereby there was prepared a toner provided with two resinous layers consisting of the second resinous layer coated on the surface of the foregoing first resinous layer. Subsequently, 10 parts by weight of the thus prepared toner was uniformly dispersed in a carrier liquid comprising Naphtha No. 6 (manufactured by ESSO Standard Sekiyu K.K.) and lsopar H (manufactured by ESSO STIANDARD Sekiyu K.K.) mixed at the weight ratio of 4 6, whereby there was obtained the desired liquid developer for electrophotography. Example 2 A mixture of 9 parts by weight of styrene and 1 part by weight of laurylmethacrylate was subjected to bulk polymerization at a temperature of 80-l00C for 6 hours within the nitrogen current in the presense of azobis-isobutyronitrile, a reaction initiator, whereby there was obtained styrene-lauryl-methacrylate copolymer (having a composition ratio of styrene resin 9: laurylmethacrylate resin 1). Next, 1 part by weight of Aniline Black (manufactured by Imperial Chemical Industries Ltd.) and 3 parts by weight of the foregoing copolymer were mixed and subjected to thermal fusion. The mixture, upon cooling to room temperature, was pulverized to about 150 mesh with a ball-mill, whereby there were obtained fine grains comprising pigment particles coated with the 1st resinous layer. Then, 5 parts by weight of the thus obtained fine grains and 2 parts by weight of butylmethacrylate homopolymer manufactured by Mitsubish Rayon Co. Ltd. and sold under the name BR 105) as the 2nd resin were mixed and subjected to thermal fusion, and, after cooling, were pulverized in the same way as above, whereby there were prepared fine grains having the second resinous layer coated on the top of the foregoing first resinous layer. Furthermore, 1 part by weight of the thus prepared fine grains, 2 parts by weight of laurylmethacrylate-hydroxymethacrylate resin copolymer (having a composition ratio of laurylmethacrylate resin 3: hydroxymethacrylate resin 1) and 5 parts by weight of a solvent comprising the aforesaid Naphtha No. 6 and lsopar H at the weight ratio of 3 7 were mixed and, after evaporation of the solvent, the mixture was pulverized in the same way as above, whereby there was prepared a toner provided with three resinous layers consisting of the 3rd resinous layer coated on the surface of the foregoing 2nd resinous layer. Subsequently, 1 part by weight of thus prepared toner was dispersed in 100 parts by weight of lsopar H, whereby there was obtained the desired liquid developer for electrophotography.

Example 3 One part by weight of Carbon Black (manufactured by Asahi Carbon Mfg. K.K. and sold under the name XG-550), 5 parts by weight of methylmethacrylate-butylmethacrylate copolymer (manufactured by Mitsubishi Rayon Co. Ltd. and sold under the name BR-95) and 5 parts by weight of toluene were mixed and heated at a temperature of 60-70C. The mixture grains obtained upon cooling after evaporation of toluene were pulverized to the grain size of less than about 150 mesh, whereby there were obtained fine grains comprising pigment particles coated with the 1st resin. A mixture of 1 part by weight of the thus obtained fine grains and 0.5 part by weight of styrene-butadiene copolymer of a molar ratio of 65 35 (manufactured by The Goodyear Tire & Rubber Co. and sold under the name Pliolite VTL) was subjected to thermal fusion and, after cooling, pulverized to the grain size of less than 150 mesh with a ball-mill, whereby there was prepared a toner. Subsequently, 10 parts by weight of the thus prepared toner was uniformly dispersed in 150 parts by weight of a carrier liquid comprising Naphtha No. 6 and lsopar H mixed at the weight ratio of 2 8, whereby there was obtained the desired liquid developer for electrophotography. Furthermore, the example was repeated using Solprene instead of Pliolite VTL, but there was obtained the same good result as the example described above.

Example 4 A dispersant comprising 1 part by weight of the fine grains having the 2nd resinous layer such as obtained in Example 2, 2 parts by weight of the 3rd resin, viz., lauryl-methacrylatehydroxymethacrylate resin copolymer of Example 2 and 10 parts by weight of a solvent consisting of Naphtha No. 6 and Isopar H mixed at the weight ratio of 7 3 was spray-dried by means of a spray-dryer to form a layer consisting of the dispersed 3rd resin on the top of the foregoing 2nd resinous layer, whereby there was prepared a toner provided with three resinotm layers, of which the outermost layer is the resindispersed layer. Subsequently, 1 part by weight of the thus prepared toner was dispersed in parts by weight of lsopar H, whereby there was obtained the desired liquid developer for electrophotography.

Next, the respective liquid developers prepared as above were applied to an electrophotographic copying material (manufactured by K.K. RICOH and sold under the name BS-1) which had been impressed with an electrostatic latent image, and development was effected thereby. As a result, the respective developers proved capable of forming quite clear copied images. Moreover, even when these liquid developers were preserved for a long period of time or were repeatedly employed for development, there was observed no coming off of the resinous layer coated on the pigment particles, and accordingly, such properties as dispersibility, polarity-control or developing efi'lciency thereof were quite effectively maintained to be the same as that at the initial stage subsequent to manufacture thereof.

What is claimed is 1. A liquid developer for electrophotography comprising 1-50 parts by weight of an electrostatically charged toner dispersed in 1000 parts by weight of a carrier liquid, said carrier liquid having an intrinsic resistivity of more than 10* 0 cm and being selected from the group consisting of an isoparaffinic hydrocarbon, a paraffinic hydrocarbon and mixtures thereof, said toner consisting of pigment particles having a first resinous layer coated on the surface of the pigment particles and a second resinous layer coated on the surface of the first resinous layer, said pigment particles being selected from the group consisting of Carbon Black (C.l.No. 77266), Spirit Black (C.l. No. 50415), Aniline Black (C.l. No. 50440), Alkali Black (C.I. No. 42750), Cyanine Blue (C.l. No. 26410), Fast Yellow (C.l. No. 13135), Fast Red (C.l No. 37125), Benzidine Orange (C.l. No. 21 and Methylene Blue (C.I. No. 25015), said first resinous layer consisting of 0.5-l0 parts by weight of an (A) resin, based on 1 part by weight of the pigment particles, said (A) resin being selected from the group consisting of polystyrene, styrene-butylmethacrylate copolymer having a molar ratio of about 7:3, styrene-laurylmethacrylate copolymer having a molar ratio of about 9:1, styrene-hydroxymethacrylate copolymer having a molar ratio of about 6:4, styrene-butadiene copolymer having a molar ratio of about 8:2 and methylmethacrylate butylmethacrylate copolymer, said second resinous layer consisting of l-20 parts by weight of a (B) resin, based on 1 part by weight of the pigment particles, said (B) resin being selected from the group consisting of styrene-laurylmethacrylate copolymer having a molar ratio of about 1:1, styrene-butylmethacrylate-acrylic acid copolymer having a molar ratio of about 3:721, butylmethacrylate homopolymer, vinyl toluene-vinyl acetate copolymer, phenolmodified alkyd resin, styrene-butadiene copolymer having a molar ratio of about 6.5:35, laurylmethacrylate-hydroxymethacrylate copolymer having a molar ratio of about 3:1 and styrene-butadiene copolymer having a molar ratio of about 1:1.

2. A liquid developer according to claim 1, wherein the carrier liquid is a mixture of isoparaffinic and parafiinic hydrocarbons and the toner consists of Spirit Black as the pigment, styrene-butylmethacrylate copolymer having a molar ratio of about 7:3 as the (A) resin and butylmethacrylate homopolymer as the (B) resin.

3. A liquid developer according to claim 1, wherein the carrier liquid is a mixture of isoparaffinic and paraffinic hydrocarbons and the toner consists of Carbon Black as the pigment, methyl methacrylate butyl methacrylate copolymer as the (A) resin and styrene butadiene copolymer having a molar ratio of about 65:35 as the (B) copolymer as the (A) resin and vinyl toluene vinyl acetate copolymer as the (B) resin. 7

5. A liquid developer for electrophotography comprising 1-50 parts by weight of an electrostatically changed toner dispersed in 1,000 parts by weight of a carrier liquid, said carrier liquid having an intrinsic resistivity of more than 10 0. cm and being selected from the group consisting of an isoparaffinic hydrocarbon, a paraffinic hydrocarbon and mixtures thereof, said toner consisting of pigment particles having a first resinous layer coated on the surface of the pigment particles, a second resinous layer coated on the surface of the first resinous layer, and a third resinous layer coated on the surface of the second resinous layer, said pigment particles being selected from the group consisting of Carbon Black (C.1.No. 77266), Spirit Black (C.1.No. 50415), Aniline Black (C.1. No. 50440), Alkali Black (C.l. No. 42750), Cyanine Blue(C.I. No. 26410), Fast Yellow (C.1. No. 13135), Fast Red (C.l.No. 37125), Benzidine Orange (C.l.No. 21110), and Methylene Blue (C.l.No. 25015), said first resinous layer consisting of 05-10 parts by weight of an (A) resin, based on 1 part by weight of the pigment particles, said (A) resin being selected from the group consisting of polystyrene, styrene-butylmethacrylate copolymer having a molar ratio of about 7:3, styrenelaurylmethacrylate copolymer having a molar ratio of about 9:1, styrene-hydroxymethacrylate copolymer having a molar ratio of about 6:4, styrene-butadiene copolymer having a molar ratio of about 8:2 and methylmethacrylate-butylcopolymer having a molar ratio of about 1:1, styrene-butylmethacrylate-acrylic acid copolymer having a molar ratio of about 327:1, butylmetliacrylate homopolymer, vinyl toluenevinyl acetate copolymer, phenol-modified alkyd resin, styrene-butadiene copolymer having a molar ratio of about 6.5:3.5, laurylmethacrylate-hydroxymethacrylate copolymer having a molar ratio of about 3:1 and styrene-butadiene copolymer having a molar ratio of about 1:1, said second resinous layer consisting of 0.5 10 parts by weight of one of said (A) and (B) resins, but differing from the resins of said first and third layers, based on 1 part by weight of the pigment particles.

6. A liquid developer according to claim 5, wherein the carrier liquid is an isoparaffinic hydrocarbon and the toner consists of Aniline Black as the pigment, styrene-lauryl methacrylate copolymer having a molar ratio of 9:1 as the first resinous layer, butyl methacrylate homopolymer as the second resinous layer and lauryl methacrylate hydroxy methacrylate copolymer having a molar ratio of about 3:1 as the third resinous layer. 

2. A liquid developer according to claim 1, wherein the carrier liquid is a mixture of isoparaffinic and paraffinic hydrocarbons and the toner consists of Spirit Black as the pigment, styrene-butylmethacrylate copolymer having a molar ratio of about 7:3 as the (A) resin and butylmethacrylate homopolymer as the (B) resin.
 3. A liquid developer according to claim 1, wherein the carrier liquid is a mixture of isoparaffinic and paraffinic hydrocarbons and the toner consists of Carbon Black as the pigment, methyl methacrylate - butyl methacrylate copolymer as the (A) resin and styrene - butadiene copolymer having a molar ratio of about 6.5: 3.5 as the (B) resin.
 4. A liquid developer according to claim 1, wherein the carrier liquid is a mixture of isoparaffinic and paraffinic hydrocarbons and the toner consists of Carbon Black as the pigment, methylmethacrylate - butylmethacrylate copolymer as the (A) resin and vinyl toluene - vinyl acetate copolymer as the (B) resin.
 5. A liquid developer for electrophotography comprising 1-50 parts by weight of an electrostatically changed toner dispersed in 1,000 parts by weight of a carrier liquid, said carrier liquid having an intrinsic resistivity of more than 109 Omega cm and being selected from the group consisting of an isoparaffinic hydrocarbon, a paraffinic hydrocarbon and mixtures thereof, said toner consisting of pigment particles having a firsT resinous layer coated on the surface of the pigment particles, a second resinous layer coated on the surface of the first resinous layer, and a third resinous layer coated on the surface of the second resinous layer, said pigment particles being selected from the group consisting of Carbon Black (C.I.No. 77266), Spirit Black (C.I.No. 50415), Aniline Black (C.I. No. 50440), Alkali Black (C.I. No. 42750), Cyanine Blue (C.I. No. 26410), Fast Yellow (C.I. No. 13135), Fast Red (C.I.No. 37125), Benzidine Orange (C.I.No. 21110), and Methylene Blue (C.I.No. 25015), said first resinous layer consisting of 0.5-10 parts by weight of an (A) resin, based on 1 part by weight of the pigment particles, said (A) resin being selected from the group consisting of polystyrene, styrene-butylmethacrylate copolymer having a molar ratio of about 7:3, styrene- laurylmethacrylate copolymer having a molar ratio of about 9:1, styrene-hydroxymethacrylate copolymer having a molar ratio of about 6:4, styrene-butadiene copolymer having a molar ratio of about 8:2 and methylmethacrylate-butylmethacrylate copolymer, said third resinous layer consisting of 1-20 parts by weight of a (B) resin, based on 1 part by weight of the pigment particles, said (B) resin being selected from the group consisting of styrene-lauryl-methacrylate copolymer having a molar ratio of about 1:1, styrene-butylmethacrylate-acrylic acid copolymer having a molar ratio of about 3:7:1, butylmethacrylate homopolymer, vinyl toluene-vinyl acetate copolymer, phenol-modified alkyd resin, styrene-butadiene copolymer having a molar ratio of about 6.5:3.5, laurylmethacrylate-hydroxymethacrylate copolymer having a molar ratio of about 3:1 and styrene-butadiene copolymer having a molar ratio of about 1:1, said second resinous layer consisting of 0.5 - 10 parts by weight of one of said (A) and (B) resins, but differing from the resins of said first and third layers, based on 1 part by weight of the pigment particles.
 6. A liquid developer according to claim 5, wherein the carrier liquid is an isoparaffinic hydrocarbon and the toner consists of Aniline Black as the pigment, styrene-lauryl methacrylate copolymer having a molar ratio of 9:1 as the first resinous layer, butyl methacrylate homopolymer as the second resinous layer and lauryl methacrylate - hydroxy methacrylate copolymer having a molar ratio of about 3:1 as the third resinous layer. 